In the field of optical communication and optical computing there is a strong demand for optical gates and switches capable of performing very fast execution of controlling and directing optical pulses. One example of such application is inside an ultra fast ADD/DROP node in an optical communication network where optical gates and switches respond rapidly to direct and re-route network traffic in the event of fiber-cut and/or node failure. Another example is in all-optical packet routing using optical gates and switches that are activated and controlled by optical signals to achieve ultra fast response in a very short latency time.
There are various types of electro-optic switches, such as, those that are based on Mach-Zehnder Interferometers (MZI). The activation of such switches is performed by changing the phase of the signal in one branch of the MZI. These switches and others that are activated by changing (shifting) the phase of one optical component of the signal suffer from the following disadvantages.
The process of changing phase includes changing optical properties of the media, by the control signal (activating signal of switch), along which the optical signal or its component propagates. Changing the optical properties of the media from which the switch is fabricated may be achieved electrically, by the control signal, using applied voltage or injected current to change the charge carrier density of the media that results in change in refractive index of the material. Changing the refractive index of the media may also be performed, by the control signal, using thermal process or mechanical pressure produced by piezoelectric crystals. These processes, used to produce phase shift of the signals in the switches, are relatively slow since their speed is limited by the time constant related to the state change of the media from which the phase shifter of the switch is made of and thus can not be used for ultra high speed switches.
Another disadvantage of these switches is related to their basic principle of operation that is based on phase shift. This principle inherently makes these switches sensitive to phase drifts. Accordingly, these switches need be controlled by a closed loop phase control, resulting in a switch that is not a stand alone device, requires external controllers, is complicated, power consuming, and expensive.
Switches that are phase insensitive and fast may be constructed using Non-linear Optical Loop Mirrors. Such a switch, also known as Terahertz Optical Asymmetric Demultiplexer (TOAD), is based on a loop mirror that includes a semiconductor optical amplifier as a phase shifter. While TOAD is fast and phase insensitive since it is activated optically, it still suffers from following three disadvantages.
First, when TOAD is constructed from optical fibers its loop is polarization sensitive due to birefringence of the optical fibers.
A second disadvantage of TOAD is its sensitivity to the pattern of the information pulses. The basic operation principle of TOAD requires that the phase shift of one optical component, propagating in the optical loop, is completed, by the control signal, before the arrival of the second optical component to the phase shifter. The TOAD's principle of operation allows its operation only with Return-to-Zero (RZ) modulation format and is not suitable for Non-Return-to-Zero (NRZ) modulation format, which is the most common format used today.
A third disadvantage of the TOAD is related to the same basic principle of operation as mentioned above. It requires the activating signal be in a form of pulses that are synchronized to the information pulses. This means that the TOAD requires a modulator or oscillator for generating the activating pulses and a clock recovery system for synchronization. All these make the TOAD a device that cannot be controlled by a Continuous Wave (CW) optical beam, is not a stand-alone device, and requires additional systems that make it expensive, complicated, and power consuming.